The present invention relates to methods and means for increasing the focal length of a zoom lens at its telephotographic side.
In the past, this has been done by providing an afocal attachment lens in front of the zoom lens system. FIG. 1 shows a means for providing telephotographic characteristics for a zoom lens system by said afocal attachment lens. Here the afocal attachment lens consists of a positive lens group 1 and a negative lens group 2. In FIG. 1 an object point at infinity is imaged on a film surface 4 by an attachment lens system (1, 2) and a zoom lens system 3. Since the attachment lens in this method is an afocal system, when the focal distance or length of the positive lens group 1 is expressed by f.sub.1, the focal distance of the negative lens group 2 is represented by f.sub.2, and the distance between the principal points of the positive lens group 1 and the negative lens group 2 is expressed by D, the result is expressed by: EQU f.sub.1 =D+.vertline.f.sub.2 .vertline.
At this time, the focal distance f.sub.z of the zoom lens system 3 is expressed by: ##EQU1## wherein the focal distance of the zoom lens at telephotographic end is expressed by f.sub.T. Since the relationship between f.sub.1 and f.sub.2 is f.sub.1 &gt;.vertline.f.sub.2 .vertline. because of the aforementioned afocal condition equation f.sub.1 =D+.vertline.f.sub.2 .vertline., f.sub.1 /.vertline.f.sub.2 .vertline. can be expressed by the inequality f.sub.1 /.vertline.f.sub.2 .vertline.&gt;1, and thus telephotographic characteristics are provided. The degree of telephotographic characteristics provided will be greater as D becomes longer and .vertline.f.sub.2 .vertline. shorter when f.sub.1 is constant, or as f.sub.1, .vertline.f.sub.2 .vertline. are made shorter when D is constant. However, an attachment lens of this type become complicated because of the need to satisfactorily correct the chromatic, spherical and other aberrations of the total lens system. Also such lenses are undesirably large and heavy, not easily portable and have poor handling characteristics.